This invention relates generally to coatings and specifically to a printable silicone-based printing varnish or ink for use with printing system such as lithography, letterpress, letterset, silk screen, etc.
There is a wide variety of uses for a printed silicone-based, hydrophobic and/or release coating. Areas can be selectively printed for water repellency and/or release properties with other areas unprinted to permit other processing incompatible with such a coating. Additionally, such a hydrophobic and/or release coating or ink which is compatible with pigmentation, dyes and the like and which is selectively printable has a wide variety of uses including over-printing. All uses to which a printable coating may be put are included within the context of the present invention although not specifically catalogued herein.
Silicone-based release coatings are well known but have not been used in printing by lithography letterpress, silk screen, or the like, all of which presently employ oil-based inks.
Oil-based printing inks and varnishes constitute a major portion of all inks and varnishes in use by the printing industry today. Such inks and varnishes employ drying oils such as linseed oil, chinawood oil, etc., in conjunction with rosin-based resins such as modified phenolics, maleic modified rosin esters, etc., and hydrocarbon resins. For heat-set inks, hydrocarbon resins and rosin-based resins with or without drying oils are dissolved in petroleum-based oils, usually straight run middle distillates or hydrotreated middle distillates. Final formulations for the actual printing inks and varnishes have been prepared by the millions, and are routinely developed by those skilled in the art. "The Printing Ink Handbook" (1980) compiled by the Product and Technical Publications Committees of the National Association of Printing Ink Manufacturers, Inc., Harrison, N.Y., provides descriptions of various aspects of printing processes and inks and a glossary of terms used in the printing ink industry. This publication is expressly incorporated herein by reference.
Conventional printing inks and varnishes are commonly characterized as falling into two broad categories, "paste inks" and "fluid inks." Paste inks are relatively high solids compositions which are viscous and tacky. Letterpress, offset lithographic, letterset, intaglio and silk screen inks are all paste inks. Fluid inks are low viscosity liquids which contain large amounts of volatile solvents. Flexographic and gravure inks are typical fluid inks.
Among paste inks, letterpress, offset lithographic and letterset inks and varnishes are relatively "long," i.e., they can be drawn out into a long thread without breaking; while silk screen and intaglio inks and varnishes are relatively "short" and "buttery," i.e., a thread cannot be drawn from them without breaking.
Paste inks and varnishes for lithographic and letterpress printing are commonly measured for viscosity and yield utilizing a falling rod viscometer in accordance with ASTM Method 4040-81 at a shear rate of 2500 sec..sup.-1
"Tack," (expressed in gram-meters) is a measure of the internal cohesion of an ink film and thus its resistance to splitting between two rapidly moving surfaces (rolls) rather than transferring from one to the other is measured in accordance with ASTM Method D4361-84 on the Inkometer, an instrument made by the Thwing-Albert Company, Philadelphia, PA. This instrument can be operated at different speeds, and is also used to determine the effect of speed (revolutions per minute) on ink characteristics such as "misting" or "flying," "spitting" and distribution of the ink or varnish on the rollers of the instrument. "Mist" is a fine mist or spray of ink or varnish thrown off the rapidly moving surfaces (rollers) of the Inkometer or a printing press. "Spit" is blobs of ink or varnish thrown off the rollers. Spitting is characteristic of excessively viscous compositions. Experience has shown that observations of misting, spitting, or poor ink distribution on the Inkometer are strongly indicative that similar behavior will be observed on a printing press at similar speeds.
These oil-based printing inks and varnishes and their primary organic components are generally incompatible with silicone oils and resins. The solubility characteristics of silicone oils and resins are so limited that a successful printing ink or varnish vehicle based on silicones must be substantially composed of silicone ingredients.
The only reactive silicone coatings applied to the surface of substrates that are similar to substrates utilized in the printing, publication, packaging, and converting industries are silicone "release" coatings.
These release coating compositions have tradionally been solvent solutions or aqueous emulsions of reactive silicone polymers, wherein the fluid composition used for application to a substrate usually contained 3% to 15% reactive silicon polymer.
The equipment used for application of these release compositions to the substrate are Mayer rod coaters, roller coaters, reverse roll coaters, air knife coaters, trailing blade coaters, gravure roll coaters and the like. These coating systems apply 100% continuous coverage to the substrate.
With the advent of "solventless" reactive silicon polymers for release coatings as described in U.S. Pat. Nos. 4,071,644, 3,922,443, 3,936,581 and 3,928,629 the preferred method of coating is by offset gravure utilizing differential roll speeds. This method applies 100% continuous coverage to the substrate.
In brief, all of the efforts to develop release coating materials have been directed to compositions which will readily afford a continuous or 100% coverage to a substrate regardless of the method of application employed. The requirement for 100% coverage directs one to compositions which have good flow properties so that flow of the coating after application will ensure that any pinholes, minor skips, etc. are covered by the post-application flow out before curing.
These requirements are different from the flow properties required for a satisfactory printing ink or varnish. Printing requires that the applied material remain exactly as printed with as little post-application flow as is possible, preferably none at all. Any significant flow out will destroy the sharpness of the image, and in multicolor printing will lead to running or mixing of one color with another at boundaries where two (or more) colors meet. In addition, the printing ink or varnish must have sufficient internal cohesion or tack to allow it to transfer as completely as possible from one surface to another in the printing process and to retain definition throughout the various steps which are required to ultimately transfer it to the substrate which is to be printed. Therefore, printed areas should have a continuous coating, and the unprinted areas left uncovered.
Thus the balance of properties for a printing ink or varnish is a far more complex matter than that required for the continuous coating processes of the prior art systems, with the added proviso that while the internal cohesion and flow-resistant properties must be high enough to allow definition to be retained in the printing process they cannot be too high, or poor distribution will occur and the ink or varnish will be unusable. The balance of properties will vary from one printing process to another, but given a vehicle which is operative according to the present invention, formulation for a particular printing system is readily accomplished by those skilled in the art.
For these reasons, the well-known silicone-based release coatings exhibit qualities which make them unsuitable for printing techniques such as offset lithography, letterpress, etc. For instance, the silicone release coating disclosed in U.S. Pat. No. 4,071,644 includes a two-part, curable, solventless composition, one part of which is a hydroxyl (silanol) chainstopped polysiloxane exhibiting preferably a viscosity of from 300 to 1000 ctsk. at 25.degree. C. and the other part of which is an organic hydrogenpolysiloxane exhibiting a viscosity of from 10 to 100 ctsk. at 25.degree. C. In another species of the invention disclosed in Example 2 in U.S. Pat. No. 4,071,644, a vinyl chainstopped polysiloxane fluid is disclosed and exhibits a viscosity of 300-1000 ctsk. at 25.degree. C.
U.S. Pat. No. 3,922,443 discloses a two-part solventless system, one part of which is a hydroxyl chainstopped diorganopolysiloxane which has a viscosity not greater than 1,000 centipoise at 20.degree. C. (the abstract states not greater than 5,000 centipoise), and the other part an organophydrogenpolysiloxane which has a viscosity not greater than 100 centipose at 20.degree. C. The specific dimethylpolysiloxane employed has a viscosity of 940 centipoise at 20.degree. C., and the formulated composition a viscosity of 740 centipoise at 20.degree. C.
U.S. Pat. No. 3,936,581 also describes a two-part solventless system based on a hydroxyl chainstopped diorganopolysiloxane and an organohydrogenpolysiloxane. The preferred diorganopolysiloxane has a viscosity not less than 50 centipoise at 20.degree. C., and the specific dimethylpolysiloxane employed has a viscosity of 140 centipoise. The patent also describes a solvent-based system which employs a dimethylpolysiloxane having a viscosity of 19,900 centipoise. Again the direction of the teaching of this patent is to a low-viscosity formulation to be applied to a substrate by prior art methods.
U.S. Pat. No. 3,900,617 discloses in Example 1 a vinyl chainstopped polysiloxane fluid having a viscosity of 4000 centistokes at 25.degree. C. which is cross-linked with hydrogenpolysiloxane as disclosed in U.S. Pat. No. 4,071,644. This is employed as an aqueous emulsion which is unsuitable for printing systems referred to above.
A quite similar two-part solventless, curable silicone release coating is disclosed in U.S. Pat. No. 3,928,629 which composition exhibits a maximum viscosity of 4000 ctsk. at 25.degree. C.
While U.S. Pat. No. 3,900,617 broadly discloses the use of aqueous emulsion employing vinyl chainstopped polysiloxanes which may exhibit viscosities up to 750,000 centistokes, the vinyl chainstopped polysiloxane specifically exemplified therein has a viscosity of 4000 ctsk. Furthermore, the aqueous emulsion is totally unsuitable for use in processes such as lithography which require the use of inks or varnishes which are immiscible with water.
All of the above compositions are based on relatively low molecular weight vinyl or hydroxyl chainstopped polydimethylsiloxanes which exhibit near-Newtonian viscosity behavior (i.e., they exhibit near constant viscosity at all but the very highest shear rates). Paste inks are characterized by non-Newtonian viscosity characteristics and exhibit a significantly higher viscosity at zero or low shear than under the high shear conditions of printing. In other words, paste inks become more fluid under shear. High viscosity at low shear means that the ink will resist static flow and retain a sharp image or definition in the printed image. Low viscosity at high shear allows good distribution of the ink or varnish on the rollers and printing plate. Therefore, the compositions of U.S. Pat. Nos. 3,900,617; 3,922,443; 3,928,629; 3,936,581 and 4,071,644 lack the properties required for paste inks and varnishes. They also exhibit a highly undesirable tendency to mist or fly excessively. Misting or flying leads to ink or varnish on undesired portions of the material being printed, on the printing press itself, and in the atmosphere and on the surfaces of the workplace. The presence of silicone oil in unwanted areas is particularly unacceptable because it can spoil not only the work in progress, but later projects as well. In addition, low viscosity fluids will run out of the ink fountains (open-bottomed troughs) used on many printing presses. Although some of these problems may be remedied by the addition of a high molecular weight silicone gum, the misting tendency cannot be suppressed by this technique.